This invention relates to a biaxially oriented polyethylene terephthalate film having a uniaxially oriented coating having low surface energy, resulting in excellent surface release characteristics and having recycleability and to a method for the production thereof.
Films prepared from biaxially oriented polyethylene terephthalate, herein referred to as xe2x80x9cpolyester filmsxe2x80x9d, are useful for a range of coating and printing applications as is well known in the art. For instance the production of labels, magnetic tapes and thermal transfer ribbons (TTR) often takes advantage of the high modulus of the polyester films and excellent coatability of the film surface. In such applications the polyester film acts as an anchoring material for the ink or coatings used to produce labels, magnetic tapes and TTR products, rendering the adhesion of the surface coating to the polyester film as being, essentially, permanent.
Often it is necessary to coat polyester films with a surface coating to improve the adhesion of the substrate to other materials like inks or subsequent surface coatings. For instance, it is well known in the art to coat polyester films with an acrylic coating to improve the adhesion of the polyester film to various solvent based inks. Such coating is often done at the point of manufacture of the polyester film or can be done as an offline step at converting.
For example, U.S. Pat. No. 5,648,159 describes the construction of a polyester base film for the production of high-resolution images made via the dry film photoresist process. The polyester film for coating the dry film resist base has requirements for a surface with particles with an Ra (centerline average roughness)  greater than 5 nM, an average particle diameter of between 0.01 xcexcm to 3.0 xcexcm, a maximum height of  less than 1.5 xcexcm and a haze value of 1.5%. Further, U.S. Pat. No. 6,037,100 describes the use of a biaxially oriented PET film as a coversheet for photoresist. The PET film has particular requirements for surface roughness as follows:
3.0 nm less than xe2x88x92Sra less than xe2x88x9250 nm
50 nm less than xe2x88x92Spv less than =500 nm
300 particles  less than =Summit Density less than =20,000 particles,
where Sra represents the mean centerline roughness, Spv represents the height from peak to valley, and Summit Density represents number of protuberances per unit area having a height of more than 1 nm.
However, there are many applications for which the adhesion to polyester film should be reduced. Such applications are often referred to as release applications in the art. However, polyester film has a relatively high surface energy. Such a high surface energy means that polyester film will readily adhere to many materials. In order to circumvent such adhesion it is known to coat polyester with a low surface energy material such as a silicone. The silicone surface then functions as the release layer from the polyester film structure.
For example, U.S. Pat. Nos. 5,728,339 and 5,672,428 describe an inline silicone coated polyethylene terephthalate (PET) film. U.S. Pat. No. 6,057,079 describes a self-winding dry film release film requiring a radiation-cured silicone coating for the PET surface to allow the film to self-wind without needing a separate release sheet. Additionally, U.S. Pat. No. 5,879,854 teaches a polyester film containing a silica particle with specific particle sizes and pore volumes for photoresist supporting applications.
Silicones, while being low surface energy materials, often encounter handling problems making them unsuitable for many applications in which a low surface energy film would be desirable. For instance, it is well known in the art that silicone materials often cyclize on polymerization, producing low molecular weight materials. Such materials are migratory and can, and do, often poison adhesive materials, rendering release values poor and unsuitable for commercial applications.
Similarly, it is well known in the art to coat waxes on polyester films to function as releasing agents. However, waxes, because of their low molecular weight, are prone to transfer and are, therefore, limited in utility as film surface materials.
It is further desirable that the releasing polyester film be recycleable. For plastic film reprocessing the term xe2x80x9crecycleabilityxe2x80x9d often refers to the ability of waste plastic, for instance, waste plastic collected from edge trim, poor roll formation or film near the mill core for instance, to be re-ground, remelted, mixed and re-extruded with fresh plastic to produce new sheet film. Discoloration of the film because of the presence of recycled components is undesirable for most applications. Other properties causing materials not to be satisfactorily recycleable include the presence of unmelted gels, an increase to the haze of the film, smoking at the sheet die, die lines and other defects. The undesirable components are not present in the novel film of this invention.
Although it is desirable to coat polyester films with low surface energy materials to create a low surface energy and releasable polyester film, the poor reprocessability of such materials is well known in the art to cause problems. Silicones, because of their inherent hydrophobic nature, are immiscible with PET. In addition to the requirements for a coating of about 0.05 to 0.10 xcexcm thickness, the immiscibility of silicones with PET makes these products non-recycleable as an inline coating for PET. Therefore, it is desirable to create a low surface energy polyester film having a release surface coating that remains recycleable.
Accordingly, it would be advantageous to provide an oriented polyester film which is recycleable and has excellent surface release characteristics. It would also be advantageous to provide such a recycleable film having reduced adhesion to inks and other coatings but which does not comprise a cyclizable silicone surface or a transferable wax surface, either of which tends to interfere with successful recycleability of the resin of which the film is made.
We have discovered that all of the foregoing advantages can be achieved by creating a releasing polyethylene terephthalate (PET) film by inline stretching of a special coating in only one direction opposite to the drawing direction of the oriented film to create a biaxially oriented polyester film having a uniaxially oriented surface coating.
In one particular aspect, the invention relates to a releasing polyester film comprising an uniaxially oriented polyethylene terephthalate film having a surface coating wherein said polyester film has a low surface energy, and wherein said coating comprises at least one component selected from the group consisting of a polyethylene emulsion, or a polyurethane dispersion, and a mixture of polyethylene emulsion and polyurethane dispersion.
In another aspect, the invention relates to a method of making a releasing polyester film comprising the steps of stretching said film to generate a uniaxially oriented film and applying to said uniaxially oriented film a coating comprising at least one component selected from the group consisting of polyethylene emulsion, polyurethane dispersion, and a mixture of polyethylene emulsion and polyurethane dispersion, and then stretching said coated film to provide a biaxially oriented film having a uniaxially oriented coating.